OP_RETURN is an opcode in the Bitcoin scripting system that marks a transaction output as invalid. When it's used, the transaction output can't be spent, which effectively burns that amount of bitcoin.

The idea behind OP_RETURN is to allow you to embed small amounts of data in the blockchain without cluttering up the UTXO (Unspent Transaction Output) set. This can be useful for things like timestamping documents or proving data existed at a certain time.

Some folks argue it's not a good use of blockchain space and could lead to bloating. But then again, others see value in its ability to embed arbitrary data on-chain.

So yeah, OP_RETURN outputs are "invalid" for spending purposes but they're still part of valid transactions that get confirmed on the blockchain!